Fiber optic cables utilized increasingly in the communication field comprise indefinite flexible lengths of a fiber optic bundle and a load-bearing strand enclosed within a tubular sheath. When installed in the field the cable must be unwound from a reel and pulled under tension through a loose-fitting outer protective tube of similar length and flexibility. The outer tube is typically of plastic and when extruded it has a pull-line of ribbon or strand disposed inside it throughout its length. One end of the pull-line is attached to an end of the fiber optic cable and particularly its load-bearing strand so that the cable can be pulled through the outer tube.
Tensile loads of up to but not exceeding 600 to 1200 pounds may be imposed on fiber optic cables during the pulling operation. Because of friction of the cable on the inside surface of the outer tube the required loading becomes progressively greater as the operation proceeds. When the maximum allowable load is reached pulling must cease and that determines the overall length of the sheathed fiber optic cable.
To reduce that friction a silicone lubricatant is sometimes disposed over the inside surface of the outer protective tube when the tube is extruded. To the same end attempts have been made to reduce the contact area between the inside of the outer tube and the exterior of the fiber optic cable sheath by forming longitudinal inwardly projecting ridges on the inside surface of the outer tube against which the cable sheath slides. However a cable sheath of circular cross section sliding within a larger protective tube of circular cross section engages the tube along an extended linear contact area which is very close to a line contact, and it does not greatly reduce that area by forming ridges on the inside of the outer tube.
The principal object of the present invention is to substantially reduce the contact area between the exterior of the fiber optic cable and the inside of the protective outer tube and thereby reduce friction when the cable is pulled through the tube. This in turn permits longer lengths of cable to be drawn into protective tubing before reaching the maximum allowable tension load which the cable can withstand.